Most people possess an automatic "gaydar" which assesses sexual orientation in less than a blink of an eye, research has shown.

In tests, scientists found that gaydar works on an unconscious level and is more accurate when directed at women.

Volunteers asked to distinguish between photos of "straight" and "gay" faces were able to do so in just 50 milliseconds - a third of the time of an eyeblink. Their accuracy remained greater than chance even when the photos were upside down.

For women's faces, participants were 65% accurate in guessing sexual orientation when the photos were briefly flashed on a computer screen. Differentiating between gay and straight men turned out to be harder. In this case, gaydar got the answer right only 57% of the time.

The research, published in the online journal Public Library of Science ONE, involved 129 college students. They were each shown 96 photos of young men and women who identified themselves as gay or straight.

US psychologist Joshua Tabak, who led the study at the University of Washington, said: "It may be similar to how we don't have to think about whether someone is a man or a woman or black or white. This information confronts us in everyday life."

Only photos of people without "give away" clues such as facial hair, make-up or piercings were used in the experiment. The photos were also cropped so that only faces, not hairstyles, were visible. When the faces were flipped upside down, gaydar accuracy slipped a little but remained statistically above chance.

The difference in accuracy for women's and men's faces was driven by more false alarm errors with photos of men. Participants were more likely to make the mistake of thinking a straight man was gay.

Mr Tabak suspects this may be because people are more familiar with the concept of gay men than with lesbians. Volunteers may therefore have taken a more liberal, pro-gay view of men's faces.

Not everyone possesses gaydar, the research suggests. There are "a small number of people with no ability to distinguish gay and straight faces" said Mr Tabak. People from older generations, or cultures where homosexuality is not openly recognised, may have more difficulty making judgments, he added.